This invention relates to electroplating processes and equipment, and more particularly to means for supporting a printed circuit board immersed in a liquid electrolyte while biasing the printed circuit board and immediate supporting structure to attract metal ions from the electrolyte, thus to form a layer of plating metal over the surface of the printed circuit board.
Electroplating of metallic layers upon printed circuit boards is a well known technique, frequently employing a container or tank holding a liquid electrolyte rich in ions of the metal to be plated. The printed circuit board to be plated is immersed in the electrolyte solution and electrically coupled as a cathode. An object composed of the plating metal also is immersed in the electrolyte and coupled as an anode to provide a source of ions to the electrolyte. During electroplating, metal ions proximate the printed circuit board combine with electrons on the surface of the circuit board and are reduced to the metal. Meanwhile, the metal object or anode is gradually ionized, continually providing ions to the electrolyte.
The plating rate depends largely upon the rate at which the metal ions may be caused to flow through the electrolyte between the anode and the printed circuit board. Irregularities in the electrical field near the circuit board, particularly at projections or sharp edges (for example at through-holes), can cause unwanted discontinuities in thickness of the electroplated layer. One attempt to counteract this effect is found in U.S. Pat. No. 4,174,261 (Pellegrino), disclosing a plated article supported above the electrolyte surface as a cathode between two anode manifolds, each with an array of spray nozzles. An array of suction openings can be intermingled with each array of spray nozzles. Further, opposing manifolds can be offset such that spray nozzles of one manifold are aligned with suction openings of the other, thereby improving the plating of through-holes by forcing electrolyte through them, due to the localized pressure differential. More generally, the combination of providing electrolyte through spray nozzles and drawing away excess electrolyte through interstitially arranged suction nozzles is said to result in a more uniform thickness in the plated metal layer including walls defining through holes.
Uniformity of thickness is enhanced by organic brightener and leveler additives, according to U.S. Pat. No. 4,667,049 (Heikkila et al), which discloses one method of manufacturing dialkylamino-thioxomethyl-thioalkanesulfonic acid compounds for use in electrolyte solutions. A somewhat related disclosure centers on the quantitative determination of organic composition concentrations in plating baths. U.S. Pat. No. 4,628,726 (Heikkila et al) discloses the use of high pressure liquid chromatography to measure concentration of organic brightener additive constituents.
Yet another factor influencing the plated layer is the nature of the rack or other framework supporting the printed circuit board within the electrolyte during plating. Conventional rack designs utilize hand-operated wing nuts for mounting the circuit board and have a minimum thickness or clearance of about four inches. Such designs feed current into the rack at its top, and depend upon the conductivity of the rack to distribute current to the printed circuit board. The lack of an even current distribution can severely affect large printed circuit boards, for example square boards measuring thirty inches on one side. Racks that extend the entire length of opposite sides of the circuit board tend to enhance current distribution, yet result in uneven plating thicknesses, in particular by increased thickness near the racks and by differences in thickness from the top to the bottom of the circuit board.
Therefore, it is an object of the present invention to provide a rack or framework for supporting large circuit boards for electroplating, and for providing current to such circuit boards for a more uniform thickness in the electroplated metal layer.
Another object is to provide a compact rack design which permits closer spacing between printed circuit boards being plated and peripheral tank equipment such as manifolds and anodes.
Yet another object is to provide a means for supplying current to at least two separate locations along a vertical side edge of a printed circuit board during electroplating, and adjusting the current at one location with respect to the current at the other location to provide a more uniform electrical field proximate the printed circuit board.